1. Field of Invention
The present invention relates to a dichroic prism, a prism unit thereof, and a projection display apparatus.
2. Description of Related Art
Most projection display apparatuses for projecting a color image onto a projection screen utilize a dichroic prism. A dichroic prism is an optical element that synthesizes and emits lights of three colors, red, green, and blue, in the same direction.
FIG. 13 is a conceptual view showing the principal part of a projection display apparatus. This projection display apparatus includes three liquid crystal light valves 42, 44, and 46, a dichroic prism 48, and a projection lens 50. Red reflecting films 48R and blue reflecting films 48B are located in the form of a cross in the center of the dichroic prism 48. The dichroic prism 48 synthesizes lights of three colors, red, green, and blue that are modulated by the three liquid crystal light valves 42, 44, and 46, and emits the lights toward the projection lens 50. The projection lens 50 focuses the synthesized lights onto a projection screen 52.
A general type of dichroic prism is made by bonding rectangular surfaces of four rectangular prisms that are equal in size. The red reflecting films 48R are previously formed on predetermined rectangular surfaces of two rectangular prisms so that they form a flat plane when four rectangular prisms are bonded together. The blue reflecting films 48B are also previously formed on predetermined rectangular surfaces of two rectangular prisms in a similar manner. In making a dichroic prism by bonding four rectangular prisms of equal size, however, it is very difficult to accurately bond the rectangular prisms so that the red reflecting films 48R form a flat plane and the blue reflecting films 48B form a flat plane.
Accordingly, in order to accurately bond rectangular prisms together, it is well known to make some of the rectangular prisms longer than the others, as shown in, for example, FIG. 1 of Japanese Unexamined Patent Publication No. 7-294845.
FIGS. 14(A) and 14(B) are explanatory views showing problems of such a conventional dichroic prism. As shown in FIG. 14(A), a dichroic prism 48 includes two rectangular prisms 61 and 62 that are long in the longitudinal direction (also referred to as “a long rectangular prism pair”), and two rectangular prisms 63 and 64 that are short in the longitudinal direction (also referred to as “a short rectangular prism pair”). Blue reflecting films 48B are formed on interface surfaces between the long rectangular prism pair 61 and 62 and the short rectangular prism pair 63 and 64. Rectangular surfaces of the long rectangular prism pair 61 and 62 are partly exposed, and the blue reflecting films 48B are also formed on the exposed surfaces. Moreover, red reflecting films are formed on an interface surface between the long rectangular prism pair 61 and 62, and on an interface surface between the short rectangular prism pair 63 and 64, respectively.
In a projection display apparatus, there is light that returns from a projection lens 50 toward the dichroic prism 48 because of reflection by the projection lens 50, or the like. A description will now be given of a case in which white return light W is produced in the example shown in FIGS. 14(A) and 14(B). FIG. 14(B) is a horizontal sectional view of protruding portions of the long rectangular prism pair 61 and 62 at the top thereof. When the return light W enters a protruding rectangular surface of the rectangular prism 61 (a surface provided with the blue reflecting film 48B), it is totally reflected by the rectangular surface. Only red light R of the totally reflected return light W is reflected by the red reflecting film 48R, and is emitted again toward the projection lens 50.
In the conventional dichroic prism shown in FIGS. 14(A) and 14(B), the return light, which returns from the light emitting surface side to the dichroic prism, is thus reflected inside the dichroic prism and is emitted again from the light emitting surface. Consequently, the influence of this return light is exerted on an image to be reproduced by light emitted from the dichroic prism.